Evidence that a circulating factor is responsible for proteinuria in FSGS includes early recurrence of proteinuria in renal allografts, favorable clinical responses to plasmapheresis and immunoadsorption therapy, and our observation that plasma from FSGS patients increases glomerular albumin permeability (Palb) following in vitro incubation of isolated glomeruli to serum or plasma of certain FSGS patients. We have defined activity in this assay by the Palb of isolated glomeruli after incubation under standard conditions as "FS activity". We have shown that FS activity of specimens obtained prior to transplantation predicts recurrence of proteinuria and later allograft loss. FS activity is diminished by plasmapheresis and a protein fraction of plasmapheresis fluid carries this activity. Intravenous injection of fractions derived from FSGS patients' plasma and carrying FS activity causes transient proteinuria in rats. FS activity of plasma or its fractions is concentration dependent. During purification, relative activity is increased by more than 33,000 fold compared to plasmapheresis fluid and the yield is more than 80 percent. The active component or "FSGS factor" is a protein, as evidenced by its properties of insolubility in organic solvents, heat-lability, and protease sensitivity. It is anionic at physiologic pH, and in the most highly enriched fractions, has an apparent molecular size of 30-50 kD. We have used the high affinity of the factor to certain carbohydrates to design a simplified protocol for its enrichment. In the proposed studies, we will purify the FSGS factor from plasma of FSGS patients to homogeneity and will determine its amino acid sequence and carbohydrate composition. Techniques to be employed to further purify the material include affinity chromatography, chromatofocusing, size exclusion chromatography, ion exchange chromatography, isoelectric focusing and affinity chromatography using monoclonal antibodies. We will compare the FSGS factor to known proteins and glycoproteins to determine whether it is a previously described for a novel mediator. We will use monoclonal antibodies to develop an immunoassay for further studies of human FSGS. Application of this information will permit early identification of patients with the most aggressive forms of the disease, and, eventually, to design specific treatment for FSGS in native kidneys and renal allografts.